1. Field of the Invention
The present invention relates to a method for modifying the surface of polymer materials by giving a hydrophilic property to the surface.
2. Description of the Prior Art
Conventional polymer materials have surfaces having poor hydrophilic properties and high electric resistance, and therefore they are easily charged in high voltage or are dirty by a slight mechanical action, for example, friction. Accordingly, their use field is very restricted. To obviate these drawbacks, it has been proposed to apply a surfactant to the surface of a polymer material or to knead a polymer material together with a surfactant. However, in this method, the surfactant merely adheres to the surface of the polymer material, and therefore the surfactant rubbs off the polymer material surface due to minute friction or to exposure to a highly humid atmosphere, or it is washed away together with water droplet adhered to the polymer material surface. As a result, the effect of the surfactant is lost in a very short period of time (Yukagaku, 14, 571 (1965) published by Oil Chemist Society; Engineering Materials, 13, No. 11, page 14, (1965)). In the latter method, the surfactant bleeds to the surface of the polymer material to make the surface sticky or to cause a blocking tendency in the surface, and further the surfactant remains in the interior of the polymer material and the effect of the surfactant does not reach the surface of the polymer material (Journal of the Chemical Society of Japan, Industrial Chemical Section, 68, 2209 (1965)).
Recently, it has been attempted to modify the surface of a polymer material by adding to the polymer material a block or graft copolymer consisting of a polymer portion having a hydrophilic property and a polymer portion having a hydrophobic property.
For example, Advances in Chem., No. 142, page 320 (1975) discloses a method, wherein an AB type block copolymer having a hydrophobic poly(N-lauroyl-ethyleneimine) portion, which is compatible with nylon fiber, and a hydrophilic poly(N-propionyl-ethyleneimine) portion is synthesized through a cationic ring-opening polymerization, and the resulting AB type block copolymer is added to the nylon fiber to give a hydrophilic property to the fiber surface. Further, J. Macromol. Sci.-Chem., A13(3), 401(1979) discloses a method, wherein an AB or ABA type block copolymer consisting of a hydrophobic polystyrene portion and a hydrophilic tetrahydrofuran portion is synthesized through anionic living polymerization and cationic living polymerization, and the block copolymer is added to polystyrene to give a hydrophilic property to the polystyrene surface.
However, the above described methods for synthesizing a block copolymer are ionic polymerization methods. Therefore, the polymerization reaction does not proceed satisfactorily due to the influence of a slight amount of water or metal ion which are apt to be incorporated into the reaction system during the polymerization step, or complicated polymerization steps are required, and hence the above described method for synthesizing the block copolymers are not suitable for industrial purposes.
Journal of the Adhesion Society of Japan, Vol. 17, No. 9, page 391 (1981) discloses a method for giving a hydrophilic property to the surface of polymethyl methacrylate, wherein a vinyl-polymerizable double bond is added to one end of the molecular chain of polymethyl methacrylate to produce a macromonomer, the macromonomer is copolymerized with hydroxyethyl methacrylate to produce a graft copolymer having a branch consisting of hydrophobic polymethyl methacrylate portion and a backbone consisting of hydrophilic polyhydroxyethyl methacrylate portion, and the graft copolymer is added to the polymethyl methacrylate. However, the graft copolymer obtained in the above described method, which consists of a hydrophilic polymer portion and a hydrophobic polymer portion, is very complicated in the synthesis method. Moreover, the molecular weight of the macromonomer must be kept to a relatively low molecular weight (number average molecular weight: 1,000-10,000) in order to maintain its reactivity for polymerization, and therefore the polymethyl methacrylate portion, which forms the branch of the graft copolymer, is an oligomer having a number average molecular weight of not larger than 10,000. Accordingly, when such graft polymer is added to polymethyl methacrylate in order to modify it, the graft copolymer cannot be always fully retained in the polymethyl methacrylate due to the low molecular weight of the polymethyl methacrylate portion in the branch, and therefore it is doubtful that the graft copolymer is permanently effective for modifying the surface of the polymethyl methacrylate.
As described above, the surface of a polymer material has not yet been fully and satisfactorily modified in an industrially easy method.
Bearing these problems in mind, the inventors have made various investigations and found out a surprising effect that, when a block copolymer consisting of a polymer portion having a hydrophilic property (hereinafter, referred to as a hydrophilic polymer portion), and a polymer portion having a compatibility with a polymer material to be modified, both of which polymer portions having been synthesized in the presence of a polymeric peroxide, is added to a polymer material, the surface of the polymer material can be industrially easily and permanently modified. As the result, the present invention has been accomplished.